Beam reference signal based narrowband channel measurement and cqi reporting

ABSTRACT

When beamforming (e.g., via a millimeter wave system (mmW)) is used for wireless communication, a base station may transmit beams that are directed to certain directions. Due to the directional nature of the beams in the mmW system, an approach to determine a beam that provides a desirable gain is studied. The apparatus may be a user equipment (UE). The apparatus receives, from a base station, a plurality of signals through a plurality of beams of the base station, each of the plurality of beams corresponding to a respective antenna port of a plurality of antenna ports of the base station, where the plurality of signals include a plurality of beam reference signals, a plurality of beam refinement reference signals, a plurality of channel state information reference signals (CSI-RSs), or a combination thereof. The apparatus performs channel estimation for each beam of the plurality of beams from the plurality of antenna ports based on the plurality of signals.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Application Ser.No. 62/324,861, entitled “BEAM REFERENCE SIGNAL BASED NARROWBAND CHANELMEASUREMENT AND CQI REPORTING” and filed on Apr. 19, 2016, and U.S.Provisional Application Ser. No. 62/335,630, entitled “BEAM REFERENCESIGNAL BASED NARROWBAND CHANEL MEASUREMENT AND CQI REPORTING” and filedon May 12, 2016, which are expressly incorporated by reference herein intheir entirety.

BACKGROUND Field

The present disclosure relates generally to communication systems, andmore particularly, to wireless communication using a narrowband wavesuch as a millimeter wave.

Background

Wireless communication systems are widely deployed to provide varioustelecommunication services such as telephony, video, data, messaging,and broadcasts. Typical wireless communication systems may employmultiple-access technologies capable of supporting communication withmultiple users by sharing available system resources. Examples of suchmultiple-access technologies include code division multiple access(CDMA) systems, time division multiple access (TDMA) systems, frequencydivision multiple access (FDMA) systems, orthogonal frequency divisionmultiple access (OFDMA) systems, single-carrier frequency divisionmultiple access (SC-FDMA) systems, and time division synchronous codedivision multiple access (TD-SCDMA) systems.

These multiple access technologies have been adopted in varioustelecommunication standards to provide a common protocol that enablesdifferent wireless devices to communicate on a municipal, national,regional, and even global level. An example telecommunication standardis Long Term Evolution (LTE). LTE is a set of enhancements to theUniversal Mobile Telecommunications System (UMTS) mobile standardpromulgated by Third Generation Partnership Project (3GPP). LTE isdesigned to support mobile broadband access through improved spectralefficiency, lowered costs, and improved services using OFDMA on thedownlink, SC-FDMA on the uplink, and multiple-input multiple-output(MIMO) antenna technology. However, as the demand for mobile broadbandaccess continues to increase, there exists a need for furtherimprovements in LTE technology. These improvements may also beapplicable to other multi-access technologies and the telecommunicationstandards that employ these technologies.

SUMMARY

The following presents a simplified summary of one or more aspects inorder to provide a basic understanding of such aspects. This summary isnot an extensive overview of all contemplated aspects, and is intendedto neither identify key or critical elements of all aspects nordelineate the scope of any or all aspects. Its sole purpose is topresent some concepts of one or more aspects in a simplified form as aprelude to the more detailed description that is presented later.

When a millimeter wave system (mmW) is used for wireless communication,a base station may transmit beams that are transmitted in certaindirections. Due to the directional nature of the beams in the mmWsystem, an approach to determine a beam direction that provides adesirable gain is desirable.

In an aspect of the disclosure, a method, a computer-readable medium,and an apparatus are provided. The apparatus may be a user equipment(UE) for wireless communication. The apparatus receives, from a basestation, a plurality of signals through a plurality of beams of the basestation, each of the plurality of beams corresponding to a respectiveantenna port of a plurality of antenna ports of the base station. In anaspect, the plurality of signals include a plurality of beam referencesignals, a plurality of beam refinement reference signals, a pluralityof channel state information reference signals (CSI-RSs), or acombination thereof. The apparatus performs channel estimation for eachbeam of the plurality of beams from the plurality of antenna ports basedon the plurality of signals.

In an aspect, the apparatus may be a UE for wireless communication. Theapparatus includes means for receiving, from a base station, a pluralityof signals through a plurality of beams of the base station, each of theplurality of beams corresponding to a respective antenna port of aplurality of antenna ports of the base station. In an aspect, theplurality of signals include a plurality of beam reference signals, aplurality of beam refinement reference signals, a plurality of CSI-RSs,or a combination thereof. The apparatus includes means for performingchannel estimation for each beam of the plurality of beams from theplurality of antenna ports based on the plurality of signals.

In an aspect, the apparatus may be a UE for wireless communicationincluding a memory and at least one processor coupled to the memory. Theat least one processor is configured to: receive, from a base station, aplurality of signals through a plurality of beams of the base station,each of the plurality of beams corresponding to a respective antennaport of a plurality of antenna ports of the base station, where theplurality of signals include a plurality of beam reference signals, aplurality of beam refinement reference signals, a plurality of CSI-RSs,or a combination thereof, and perform channel estimation for each beamof the plurality of beams from the plurality of antenna ports based onthe plurality of signals.

In an aspect, a computer-readable medium storing computer executablecode for wireless communications by a UE, includes code to: receive,from a base station, a plurality of signals through a plurality of beamsof the base station, each of the plurality of beams corresponding to arespective antenna port of a plurality of antenna ports of the basestation, where the plurality of signals include a plurality of beamreference signals, a plurality of beam refinement reference signals, aplurality of CSI-RSs, or a combination thereof, and perform channelestimation for each beam of the plurality of beams from the plurality ofantenna ports based on the plurality of signals.

In another aspect of the disclosure, a method, a computer-readablemedium, and an apparatus are provided. The apparatus may be a basestation for wireless communication. The apparatus transmits, to a UE, aplurality of signals through a plurality of beams of the base station,each of the plurality of beams corresponding to a respective antennaport of a plurality of antenna ports of the base station. In an aspect,the plurality of signals include a plurality of beam reference signals,a plurality of beam refinement reference signals, a plurality ofCSI-RSs, or a combination thereof. The apparatus receives, from the UE,a feedback signal including channel estimation for one or more beamsselected from the plurality of beams within one or more frequency bands.

In an aspect, the apparatus may be a base station for wirelesscommunication. The apparatus includes means for transmitting, to a UE, aplurality of signals through a plurality of beams of the base station,each of the plurality of beams corresponding to a respective antennaport of a plurality of antenna ports of the base station. In an aspect,the plurality of signals include a plurality of beam reference signals,a plurality of beam refinement reference signals, a plurality ofCSI-RSs, or a combination thereof. The apparatus includes means forreceiving, from the UE, a feedback signal including channel estimationfor one or more beams selected from the plurality of beams within one ormore frequency bands.

In an aspect, the apparatus may be a base station for wirelesscommunication including a memory and at least one processor coupled tothe memory. The at least one processor is configured to: transmit, to aUE, a plurality of signals through a plurality of beams of the basestation, each of the plurality of beams corresponding to a respectiveantenna port of a plurality of antenna ports of the base station, wherethe plurality of signals include a plurality of beam reference signals,a plurality of beam refinement reference signals, a plurality ofCSI-RSs, or a combination thereof, and receive, from the UE, a feedbacksignal including channel estimation for one or more beams selected fromthe plurality of beams within one or more frequency bands.

In an aspect, a computer-readable medium storing computer executablecode for wireless communications by a base station, includes code to:transmit, to a UE, a plurality of signals through a plurality of beamsof the base station, each of the plurality of beams corresponding to arespective antenna port of a plurality of antenna ports of the basestation, where the plurality of signals include a plurality of beamreference signals, a plurality of beam refinement reference signals, aplurality of CSI-RSs, or a combination thereof, and receive, from theUE, a feedback signal including channel estimation for one or more beamsselected from the plurality of beams within one or more frequency bands.

To the accomplishment of the foregoing and related ends, the one or moreaspects comprise the features hereinafter fully described andparticularly pointed out in the claims. The following description andthe annexed drawings set forth in detail certain illustrative featuresof the one or more aspects. These features are indicative, however, ofbut a few of the various ways in which the principles of various aspectsmay be employed, and this description is intended to include all suchaspects and their equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an example of a wireless communicationssystem and an access network.

FIGS. 2A, 2B, 2C, and 2D are diagrams illustrating LTE examples of a DLframe structure, DL channels within the DL frame structure, an UL framestructure, and UL channels within the UL frame structure, respectively.

FIG. 3 is a diagram illustrating an example of an evolved Node B (eNB)and user equipment (UE) in an access network.

FIG. 4A is an example diagram illustrating transmission of beams in onesymbol.

FIG. 4B is an example diagram illustrating transmission of beams inanother symbol.

FIG. 5 is an example diagram illustrating a subframe structure forsynchronization in a millimeter wave communication system.

FIG. 6 is an example diagram illustrating resource block usage withinone symbol in a subframe for millimeter wave communication.

FIG. 7A is an example diagram illustrating a subframe structure when afeedback signal is sent via a physical uplink control channel (PUCCH).

FIG. 7B is an example diagram illustrating a subframe structure when afeedback signal is sent via a physical uplink shared channel (PUSCH).

FIGS. 8A-8D are example diagrams illustrating a process of channelestimation based on beam reference signals (BRSs) and beam referencerefinement signals (BRRSs), according to an aspect of the disclosure.

FIG. 9 is an example diagram illustrating a subframe structure fortransmitting a BRRS.

FIG. 10 is an example diagram illustrating a subframe structure fortransmitting channel state information reference signals (CSI-RSs).

FIG. 11 is an example diagram illustrating communication between a userequipment and a base station in millimeter wave communication, accordingto an aspect of the disclosure.

FIG. 12 is a flowchart of a method of wireless communication.

FIG. 13 is a flowchart of a method of wireless communication, expandingfrom the flowchart of FIG. 12, according to an aspect.

FIG. 14 is a conceptual data flow diagram illustrating the data flowbetween different means/components in an exemplary apparatus.

FIG. 15 is a diagram illustrating an example of a hardwareimplementation for an apparatus employing a processing system.

FIG. 16 is a flowchart of a method of wireless communication.

FIG. 17 is a conceptual data flow diagram illustrating the data flowbetween different means/components in an exemplary apparatus.

FIG. 18 is a diagram illustrating an example of a hardwareimplementation for an apparatus employing a processing system.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of various configurations and isnot intended to represent the only configurations in which the conceptsdescribed herein may be practiced. The detailed description includesspecific details for the purpose of providing a thorough understandingof various concepts. However, it will be apparent to those skilled inthe art that these concepts may be practiced without these specificdetails. In some instances, well known structures and components areshown in block diagram form in order to avoid obscuring such concepts.

Several aspects of telecommunication systems will now be presented withreference to various apparatus and methods. These apparatus and methodswill be described in the following detailed description and illustratedin the accompanying drawings by various blocks, components, circuits,processes, algorithms, etc. (collectively referred to as “elements”).These elements may be implemented using electronic hardware, computersoftware, or any combination thereof. Whether such elements areimplemented as hardware or software depends upon the particularapplication and design constraints imposed on the overall system.

By way of example, an element, or any portion of an element, or anycombination of elements may be implemented as a “processing system” thatincludes one or more processors. Examples of processors includemicroprocessors, microcontrollers, graphics processing units (GPUs),central processing units (CPUs), application processors, digital signalprocessors (DSPs), reduced instruction set computing (RISC) processors,systems on a chip (SoC), baseband processors, field programmable gatearrays (FPGAs), programmable logic devices (PLDs), state machines, gatedlogic, discrete hardware circuits, and other suitable hardwareconfigured to perform the various functionality described throughoutthis disclosure. One or more processors in the processing system mayexecute software. Software shall be construed broadly to meaninstructions, instruction sets, code, code segments, program code,programs, subprograms, software components, applications, softwareapplications, software packages, routines, subroutines, objects,executables, threads of execution, procedures, functions, etc., whetherreferred to as software, firmware, middleware, microcode, hardwaredescription language, or otherwise.

Accordingly, in one or more example embodiments, the functions describedmay be implemented in hardware, software, or any combination thereof. Ifimplemented in software, the functions may be stored on or encoded asone or more instructions or code on a computer-readable medium.Computer-readable media includes computer storage media. Storage mediamay be any available media that can be accessed by a computer. By way ofexample, and not limitation, such computer-readable media can comprise arandom-access memory (RAM), a read-only memory (ROM), an electricallyerasable programmable ROM (EEPROM), optical disk storage, magnetic diskstorage, other magnetic storage devices, combinations of theaforementioned types of computer-readable media, or any other mediumthat can be used to store computer executable code in the form ofinstructions or data structures that can be accessed by a computer.

FIG. 1 is a diagram illustrating an example of a wireless communicationssystem and an access network 100. The wireless communications system(also referred to as a wireless wide area network (WWAN)) includes basestations 102, UEs 104, and an Evolved Packet Core (EPC) 160. The basestations 102 may include macro cells (high power cellular base station)and/or small cells (low power cellular base station). The macro cellsinclude eNBs. The small cells include femtocells, picocells, andmicrocells.

The base stations 102 (collectively referred to as Evolved UniversalMobile Telecommunications System (UMTS) Terrestrial Radio Access Network(E-UTRAN)) interface with the EPC 160 through backhaul links 132 (e.g.,S1 interface). In addition to other functions, the base stations 102 mayperform one or more of the following functions: transfer of user data,radio channel ciphering and deciphering, integrity protection, headercompression, mobility control functions (e.g., handover, dualconnectivity), inter-cell interference coordination, connection setupand release, load balancing, distribution for non-access stratum (NAS)messages, NAS node selection, synchronization, radio access network(RAN) sharing, multimedia broadcast multicast service (MBMS), subscriberand equipment trace, RAN information management (RIM), paging,positioning, and delivery of warning messages. The base stations 102 maycommunicate directly or indirectly (e.g., through the EPC 160) with eachother over backhaul links 134 (e.g., X2 interface). The backhaul links134 may be wired or wireless.

The base stations 102 may wirelessly communicate with the UEs 104. Eachof the base stations 102 may provide communication coverage for arespective geographic coverage area 110. There may be overlappinggeographic coverage areas 110. For example, the small cell 102′ may havea coverage area 110′ that overlaps the coverage area 110 of one or moremacro base stations 102. A network that includes both small cell andmacro cells may be known as a heterogeneous network. A heterogeneousnetwork may also include Home Evolved Node Bs (eNBs) (HeNBs), which mayprovide service to a restricted group known as a closed subscriber group(CSG). The communication links 120 between the base stations 102 and theUEs 104 may include uplink (UL) (also referred to as reverse link)transmissions from a UE 104 to a base station 102 and/or downlink (DL)(also referred to as forward link) transmissions from a base station 102to a UE 104. The communication links 120 may use MIMO antennatechnology, including spatial multiplexing, beamforming, and/or transmitdiversity. The communication links may be through one or more carriers.The base stations 102/UEs 104 may use spectrum up to Y MHz (e.g., 5, 10,15, 20 MHz) bandwidth per carrier allocated in a carrier aggregation ofup to a total of Yx MHz (x component carriers) used for transmission ineach direction. The carriers may or may not be adjacent to each other.Allocation of carriers may be asymmetric with respect to DL and UL(e.g., more or less carriers may be allocated for DL than for UL). Thecomponent carriers may include a primary component carrier and one ormore secondary component carriers. A primary component carrier may bereferred to as a primary cell (PCell) and a secondary component carriermay be referred to as a secondary cell (SCell).

The wireless communications system may further include a Wi-Fi accesspoint (AP) 150 in communication with Wi-Fi stations (STAs) 152 viacommunication links 154 in a 5 GHz unlicensed frequency spectrum. Whencommunicating in an unlicensed frequency spectrum, the STAs 152/AP 150may perform a clear channel assessment (CCA) prior to communicating inorder to determine whether the channel is available.

The small cell 102′ may operate in a licensed and/or an unlicensedfrequency spectrum. When operating in an unlicensed frequency spectrum,the small cell 102′ may employ LTE and use the same 5 GHz unlicensedfrequency spectrum as used by the Wi-Fi AP 150. The small cell 102′,employing LTE in an unlicensed frequency spectrum, may boost coverage toand/or increase capacity of the access network. LTE in an unlicensedspectrum may be referred to as LTE-unlicensed (LTE-U), licensed assistedaccess (LAA), or MuLTEfire.

The millimeter wave (mmW) base station 180 may operate in mmWfrequencies and/or near mmW frequencies in communication with the UE182. Extremely high frequency (EHF) is part of the RF in theelectromagnetic spectrum. EHF has a range of 30 GHz to 300 GHz and awavelength between 1 millimeter and 10 millimeters. Radio waves in theband may be referred to as a millimeter wave. Near mmW may extend downto a frequency of 3 GHz with a wavelength of 100 millimeters. The superhigh frequency (SHF) band extends between 3 GHz and 30 GHz, alsoreferred to as centimeter wave. Communications using the mmW/near mmWradio frequency band has extremely high path loss and a short range. ThemmW base station 180 may utilize beamforming 184 with the UE 182 tocompensate for the extremely high path loss and short range.

The EPC 160 may include a Mobility Management Entity (MME) 162, otherMMEs 164, a Serving Gateway 166, a Multimedia Broadcast MulticastService (MBMS) Gateway 168, a Broadcast Multicast Service Center (BM-SC)170, and a Packet Data Network (PDN) Gateway 172. The MME 162 may be incommunication with a Home Subscriber Server (HSS) 174. The MME 162 isthe control node that processes the signaling between the UEs 104 andthe EPC 160. Generally, the MME 162 provides bearer and connectionmanagement. All user Internet protocol (IP) packets are transferredthrough the Serving Gateway 166, which itself is connected to the PDNGateway 172. The PDN Gateway 172 provides UE IP address allocation aswell as other functions. The PDN Gateway 172 and the BM-SC 170 areconnected to the IP Services 176. The IP Services 176 may include theInternet, an intranet, an IP Multimedia Subsystem (IMS), a PS StreamingService (PSS), and/or other IP services. The BM-SC 170 may providefunctions for MBMS user service provisioning and delivery. The BM-SC 170may serve as an entry point for content provider MBMS transmission, maybe used to authorize and initiate MBMS Bearer Services within a publicland mobile network (PLMN), and may be used to schedule MBMStransmissions. The MBMS Gateway 168 may be used to distribute MBMStraffic to the base stations 102 belonging to a Multicast BroadcastSingle Frequency Network (MBSFN) area broadcasting a particular service,and may be responsible for session management (start/stop) and forcollecting eMBMS related charging information.

The base station may also be referred to as a Node B, evolved Node B(eNB), an access point, a base transceiver station, a radio basestation, a radio transceiver, a transceiver function, a basic serviceset (BSS), an extended service set (ESS), or some other suitableterminology. The base station 102 provides an access point to the EPC160 for a UE 104. Examples of UEs 104 include a cellular phone, a smartphone, a session initiation protocol (SIP) phone, a laptop, a personaldigital assistant (PDA), a satellite radio, a global positioning system,a multimedia device, a video device, a digital audio player (e.g., MP3player), a camera, a game console, a tablet, a smart device, a wearabledevice, or any other similar functioning device. The UE 104 may also bereferred to as a station, a mobile station, a subscriber station, amobile unit, a subscriber unit, a wireless unit, a remote unit, a mobiledevice, a wireless device, a wireless communications device, a remotedevice, a mobile subscriber station, an access terminal, a mobileterminal, a wireless terminal, a remote terminal, a handset, a useragent, a mobile client, a client, or some other suitable terminology.

Referring again to FIG. 1, in certain aspects, the eNB 102 may beconfigured to transmit beam reference signals for different antennaports, and the UE 104 may be configured to perform narrowband channelestimation for beams corresponding to respective antenna ports based onthe beam reference signals, and to transmit a feedback signal to the eNB102 with narrowband channel estimation information (198).

FIG. 2A is a diagram 200 illustrating an example of a DL frame structurein LTE. FIG. 2B is a diagram 230 illustrating an example of channelswithin the DL frame structure in LTE. FIG. 2C is a diagram 250illustrating an example of an UL frame structure in LTE. FIG. 2D is adiagram 280 illustrating an example of channels within the UL framestructure in LTE. Other wireless communication technologies may have adifferent frame structure and/or different channels. In LTE, a frame (10ms) may be divided into 10 equally sized subframes. Each subframe mayinclude two consecutive time slots. A resource grid may be used torepresent the two time slots, each time slot including one or more timeconcurrent resource blocks (RBs) (also referred to as physical RBs(PRBs)). The resource grid is divided into multiple resource elements(REs). In LTE, for a normal cyclic prefix, an RB contains 12 consecutivesubcarriers in the frequency domain and 7 consecutive symbols (for DL,OFDM symbols; for UL, SC-FDMA symbols) in the time domain, for a totalof 84 REs. For an extended cyclic prefix, an RB contains 12 consecutivesubcarriers in the frequency domain and 6 consecutive symbols in thetime domain, for a total of 72 REs. The number of bits carried by eachRE depends on the modulation scheme.

As illustrated in FIG. 2A, some of the REs carry DL reference (pilot)signals (DL-RS) for channel estimation at the UE. The DL-RS may includecell-specific reference signals (CRS) (also sometimes called common RS),UE-specific reference signals (UE-RS), and channel state informationreference signals (CSI-RS). FIG. 2A illustrates CRS for antenna ports 0,1, 2, and 3 (indicated as R₀, R₁, R₂, and R₃, respectively), UE-RS forantenna port 5 (indicated as R₅), and CSI-RS for antenna port 15(indicated as R). FIG. 2B illustrates an example of various channelswithin a DL subframe of a frame. The physical control format indicatorchannel (PCFICH) is within symbol 0 of slot 0, and carries a controlformat indicator (CFI) that indicates whether the physical downlinkcontrol channel (PDCCH) occupies 1, 2, or 3 symbols (FIG. 2B illustratesa PDCCH that occupies 3 symbols). The PDCCH carries downlink controlinformation (DCI) within one or more control channel elements (CCEs),each CCE including nine RE groups (REGs), each REG including fourconsecutive REs in an OFDM symbol. A UE may be configured with aUE-specific enhanced PDCCH (ePDCCH) that also carries DCI. The ePDCCHmay have 2, 4, or 8 RB pairs (FIG. 2B shows two RB pairs, each subsetincluding one RB pair). The physical hybrid automatic repeat request(ARQ) (HARQ) indicator channel (PHICH) is also within symbol 0 of slot 0and carries the HARQ indicator (HI) that indicates HARQ acknowledgement(ACK)/negative ACK (NACK) feedback based on the physical uplink sharedchannel (PUSCH). The primary synchronization channel (PSCH) is withinsymbol 6 of slot 0 within subframes 0 and 5 of a frame, and carries aprimary synchronization signal (PSS) that is used by a UE to determinesubframe timing and a physical layer identity. The secondarysynchronization channel (SSCH) is within symbol 5 of slot 0 withinsubframes 0 and 5 of a frame, and carries a secondary synchronizationsignal (SSS) that is used by a UE to determine a physical layer cellidentity group number. Based on the physical layer identity and thephysical layer cell identity group number, the UE can determine aphysical cell identifier (PCI). Based on the PCI, the UE can determinethe locations of the aforementioned DL-RS. The physical broadcastchannel (PBCH) is within symbols 0, 1, 2, 3 of slot 1 of subframe 0 of aframe, and carries a master information block (MIB). The MIB provides anumber of RBs in the DL system bandwidth, a PHICH configuration, and asystem frame number (SFN). The physical downlink shared channel (PDSCH)carries user data, broadcast system information not transmitted throughthe PBCH such as system information blocks (SIBs), and paging messages.

As illustrated in FIG. 2C, some of the REs carry demodulation referencesignals (DM-RS) for channel estimation at the eNB. The UE mayadditionally transmit sounding reference signals (SRS) in the lastsymbol of a subframe. The SRS may have a comb structure, and a UE maytransmit SRS on one of the combs. The SRS may be used by an eNB forchannel quality estimation to enable frequency-dependent scheduling onthe UL. FIG. 2D illustrates an example of various channels within an ULsubframe of a frame. A physical random access channel (PRACH) may bewithin one or more subframes within a frame based on the PRACHconfiguration. The PRACH may include six consecutive RB pairs within asubframe. The PRACH allows the UE to perform initial system access andachieve UL synchronization. A physical uplink control channel (PUCCH)may be located on edges of the UL system bandwidth. The PUCCH carriesuplink control information (UCI), such as scheduling requests, a channelquality indicator (CQI), a precoding matrix indicator (PMI), a rankindicator (RI), and HARQ ACK/NACK feedback. The PUSCH carries data, andmay additionally be used to carry a buffer status report (BSR), a powerheadroom report (PHR), and/or UCI.

FIG. 3 is a block diagram of an eNB 310 in communication with a UE 350in an access network. In the DL, IP packets from the EPC 160 may beprovided to a controller/processor 375. The controller/processor 375implements layer 3 and layer 2 functionality. Layer 3 includes a radioresource control (RRC) layer, and layer 2 includes a packet dataconvergence protocol (PDCP) layer, a radio link control (RLC) layer, anda medium access control (MAC) layer. The controller/processor 375provides RRC layer functionality associated with broadcasting of systeminformation (e.g., MIB, SIBs), RRC connection control (e.g., RRCconnection paging, RRC connection establishment, RRC connectionmodification, and RRC connection release), inter radio access technology(RAT) mobility, and measurement configuration for UE measurementreporting; PDCP layer functionality associated with headercompression/decompression, security (ciphering, deciphering, integrityprotection, integrity verification), and handover support functions; RLClayer functionality associated with the transfer of upper layer packetdata units (PDUs), error correction through ARQ, concatenation,segmentation, and reassembly of RLC service data units (SDUs),re-segmentation of RLC data PDUs, and reordering of RLC data PDUs; andMAC layer functionality associated with mapping between logical channelsand transport channels, multiplexing of MAC SDUs onto transport blocks(TBs), demultiplexing of MAC SDUs from TBs, scheduling informationreporting, error correction through HARQ, priority handling, and logicalchannel prioritization.

The transmit (TX) processor 316 and the receive (RX) processor 370implement layer 1 functionality associated with various signalprocessing functions. Layer 1, which includes a physical (PHY) layer,may include error detection on the transport channels, forward errorcorrection (FEC) coding/decoding of the transport channels,interleaving, rate matching, mapping onto physical channels,modulation/demodulation of physical channels, and MIMO antennaprocessing. The TX processor 316 handles mapping to signalconstellations based on various modulation schemes (e.g., binaryphase-shift keying (BPSK), quadrature phase-shift keying (QPSK),M-phase-shift keying (M-PSK), M-quadrature amplitude modulation(M-QAM)). The coded and modulated symbols may then be split intoparallel streams. Each stream may then be mapped to an OFDM subcarrier,multiplexed with a reference signal (e.g., pilot) in the time and/orfrequency domain, and then combined together using an Inverse FastFourier Transform (IFFT) to produce a physical channel carrying a timedomain OFDM symbol stream. The OFDM stream is spatially precoded toproduce multiple spatial streams. Channel estimates from a channelestimator 374 may be used to determine the coding and modulation scheme,as well as for spatial processing. The channel estimate may be derivedfrom a reference signal and/or channel condition feedback transmitted bythe UE 350. Each spatial stream may then be provided to a differentantenna 320 via a separate transmitter 318TX. Each transmitter 318TX maymodulate an RF carrier with a respective spatial stream fortransmission.

At the UE 350, each receiver 354RX receives a signal through itsrespective antenna 352. Each receiver 354RX recovers informationmodulated onto an RF carrier and provides the information to the receive(RX) processor 356. The TX processor 368 and the RX processor 356implement layer 1 functionality associated with various signalprocessing functions. The RX processor 356 may perform spatialprocessing on the information to recover any spatial streams destinedfor the UE 350. If multiple spatial streams are destined for the UE 350,they may be combined by the RX processor 356 into a single OFDM symbolstream. The RX processor 356 then converts the OFDM symbol stream fromthe time-domain to the frequency domain using a Fast Fourier Transform(FFT). The frequency domain signal comprises a separate OFDM symbolstream for each subcarrier of the OFDM signal. The symbols on eachsubcarrier, and the reference signal, are recovered and demodulated bydetermining the most likely signal constellation points transmitted bythe eNB 310. These soft decisions may be based on channel estimatescomputed by the channel estimator 358. The soft decisions are thendecoded and deinterleaved to recover the data and control signals thatwere originally transmitted by the eNB 310 on the physical channel. Thedata and control signals are then provided to the controller/processor359, which implements layer 3 and layer 2 functionality.

The controller/processor 359 can be associated with a memory 360 thatstores program codes and data. The memory 360 may be referred to as acomputer-readable medium. In the UL, the controller/processor 359provides demultiplexing between transport and logical channels, packetreassembly, deciphering, header decompression, and control signalprocessing to recover IP packets from the EPC 160. Thecontroller/processor 359 is also responsible for error detection usingan ACK and/or NACK protocol to support HARQ operations.

Similar to the functionality described in connection with the DLtransmission by the eNB 310, the controller/processor 359 provides RRClayer functionality associated with system information (e.g., MIB, SIBs)acquisition, RRC connections, and measurement reporting; PDCP layerfunctionality associated with header compression/decompression, andsecurity (ciphering, deciphering, integrity protection, integrityverification); RLC layer functionality associated with the transfer ofupper layer PDUs, error correction through ARQ, concatenation,segmentation, and reassembly of RLC SDUs, re-segmentation of RLC dataPDUs, and reordering of RLC data PDUs; and MAC layer functionalityassociated with mapping between logical channels and transport channels,multiplexing of MAC SDUs onto TBs, demultiplexing of MAC SDUs from TBs,scheduling information reporting, error correction through HARQ,priority handling, and logical channel prioritization.

Channel estimates derived by a channel estimator 358 from a referencesignal or feedback transmitted by the eNB 310 may be used by the TXprocessor 368 to select the appropriate coding and modulation schemes,and to facilitate spatial processing. The spatial streams generated bythe TX processor 368 may be provided to different antenna 352 viaseparate transmitters 354TX. Each transmitter 354TX may modulate an RFcarrier with a respective spatial stream for transmission.

The UL transmission is processed at the eNB 310 in a manner similar tothat described in connection with the receiver function at the UE 350.Each receiver 318RX receives a signal through its respective antenna320. Each receiver 318RX recovers information modulated onto an RFcarrier and provides the information to a RX processor 370.

The controller/processor 375 can be associated with a memory 376 thatstores program codes and data. The memory 376 may be referred to as acomputer-readable medium. In the UL, the controller/processor 375provides demultiplexing between transport and logical channels, packetreassembly, deciphering, header decompression, control signal processingto recover IP packets from the UE 350. IP packets from thecontroller/processor 375 may be provided to the EPC 160. Thecontroller/processor 375 is also responsible for error detection usingan ACK and/or NACK protocol to support HARQ operations.

Wireless communication systems may employ narrow bandwidths and highfrequency carriers. For example an mmW system may be utilized forwireless communication at a high transmission rate (e.g., transmittingfrequently). In mmW systems, when the carrier frequency is high (e.g.,28 GHz), path loss may be high. For example, the carrier frequency formmW communication may be 10 times higher than a carrier frequency forother types of wireless communication. Thus, for example, the mmW systemmay experience path loss that is approximately 20 dB higher than othertypes of wireless communication cases at lower frequencies. To mitigatethe higher path loss in mmW systems, a base station may performtransmission in a directional manner by beam-forming the transmission tofocus the transmission in a particular direction.

If the carrier frequency for wireless communication is a higherfrequency, the wavelength of the carrier is shorter. A shorterwavelength may allow a higher number of antennas to be implementedwithin a given antenna array length than a number of antennas that canbe implemented when a lower carrier frequency is used. Therefore, in themmW system (using a higher carrier frequency), a higher number ofantennas may be used in a base station and/or a UE. For example, the BSmay have 128 or 256 antennas and the UE may have 8, 16 or 24 antennas.With the higher number of antennas, a beam-forming technique may be usedto digitally change the direction of a beam by applying different phasesto different antennas. Because beam-forming in an mmW system may providea narrow beam with increased gain at the receiver, the base station mayutilize the narrow beam to transmit a synchronization signal in variousdirections using multiple narrow beams to provide coverage over a widerarea.

One challenge in using beam-forming for a mmW system arises from thedirectional nature of a beam-formed beam. In such a case, for a UE toobtain a desirable gain, the base station needs to point the beamdirectly at the UE such that the direction of the beam aligns with thelocation of the UE. If the direction of the beam is not alignedproperly, the antenna gain at the UE may be undesirably low (e.g.,resulting in low SNR, high block error rates, etc.). Further, when theUE enters the mmW system and receives transmitted data from the basestation over the mmW, the base station should be able to determine thebest beam(s) for mmW communication with the particular UE. Thus, thebase station may transmit beam reference signals (BRSs) in variousdirections via corresponding beams so that the UE may identify the bestbeam of the one or more beams received from the base station based onmeasurements on the BRSs. In the mmW communication, the base station mayalso transmit a primary synchronization signal (PSS), a secondarysynchronization signal (SSS), an extended synchronization signal (ESS),and PBCH signals for synchronization and for broadcasting systeminformation. In the mmW communication, such signals may be transmitteddirectionally via multiple beams.

If there are multiple antenna ports (multiple sets of antennas) in thebase station, the base station may transmit multiple beams per symbol.For example, the base station may use multiple antenna ports in a cellspecific manner in a first symbol of a synchronization subframe to sweepin multiple directions. The base station may then sweep in multipledirections using the multiple antenna ports in a cell specific manner inanother symbol of the synchronization sub-frame. Each antenna port mayinclude a set of antennas. For example, an antenna port including a setof antennas (e.g., 64 antennas) may transmit one beam, and multipleantenna ports may transmit multiple beams respectively, each in adifferent direction. Thus, if there are four antenna ports, the fourantenna ports may sweep through four directions (e.g., transmit fourbeams in four different directions). FIGS. 4A and 4B show examplediagrams illustrating the base station sweeping in multiple directionsin a first symbol and a second symbol, respectively. As shown in FIGS.4A and 4B, the base station may sweep in different directions in in eachsymbol, e.g., the angular/directional range of the beams for the examplein FIG. 4A is different from the angular/directional range of the beamsfor the example in FIG. 4B. FIG. 4A is an example diagram 400illustrating transmission of beams in a first symbol. A base station 402in the example diagram 400 has four antenna ports, and may transmit fourbeams 412, 414, 416, and 418 in four different directions in the firstsymbol. FIG. 4B is an example diagram 450 illustrating transmission ofbeams in a second symbol. Since the base station 402 has four antennaports, four beams 462, 464, 466, and 468 may be transmitted in fourdifferent directions in the second symbol. The beams transmitted by thebase station during the same symbol may not be adjacent with each other.

FIG. 5 is an example diagram 500 illustrating a synchronization subframestructure for a millimeter wave communication system. Thesynchronization subframe may be divided into 14 symbols, e.g., fromsymbol 0 to symbol 13. Within each symbol, 100 subcarriers may betransmitted, where the first 41 RBs are used to carry BRSs and PBCHs,the next 18 RBs may be used to carry an SSS, a PSS, and an ESS, and thenext 41 RBs may be used to carry BRSs and PBCHs.

The beam transmitted by each antenna port may change from symbol tosymbol. As discussed above, for example, for a first symbol, four beamsfrom four antenna ports of the base station may cover a first angularrange (e.g., as illustrated in FIG. 4A), while four beams from the fourantenna ports may cover a second angular range for a different symbol(e.g., as illustrated in FIG. 4B). For example, the base station mayhave 1, 2, 4, or 8 active antenna ports. Within each symbol, the basestation may transmit, to the UE, one or more of a PSS, an SSS, an ESS, aPBCH, and a BRS on each subcarrier. Each of the PSS, the ESS, the SSS,and the PBCH may be transmitted by all antenna ports of the base stationon the same subcarriers throughout different symbols of thesynchronization subframe. The PSS and SSS may be used to obtain the cellidentity and the subframe level synchronization. However, PSS and SSSmay not provide sufficient information to identify a symbol of thesubframe. Therefore, the ESS may be used to indicate a particularsymbol. The contents of the ESS may change from symbol to symbol.Therefore, the ESS may be used to indicate a symbol, in order to enablethe UE to identify a particular symbol index within the subframe. Forexample, for each received beam at the UE, the UE may identify thereceived beam based on a BRS received from the base station via thereceived beam, and may identify a symbol for the received beam based onan ESS received via the received beam. The ESS may be similar instructure with other synchronization signals such as the PSS and theSSS. For example, the ESS as well as the PSS may be based on a ZadoffChu sequence (e.g., a Zadoff Chu sequence with length 71). However,unlike the PSS, the Zadoff Chu sequence of each ESS may be cyclicallyshifted by a different amount, depending on the particular symbol. Forexample, for each different symbol, the base station cyclically shiftsthe Zadoff Chu sequence by a different amount to generate a differentESS for each different symbol. When the UE receives the ESS, the UE maydetermine the symbol index based on the amount of the cyclic shift ofthe Zadoff Chu sequence of the ESS. If more than one base station, eachin different cells, transmit ESSs, the UE may not be able to determinewhich base station transmitted the ESS. Thus, the Zadoff Chu sequence inthe ESS may include cell-specific roots (e.g., for the correspondingbase station) that are specific to a particular cell. The cell-specificroots, may enable the UE to identify which base station transmitted theESS. The Zadoff Chu sequence in the ESS may also be scrambled using acell-specific sequence, such that the UE may be able to identify whichbase station transmitted the ESS, based on the cell-specific sequence.

In an aspect, the angular space of the coverage area of a cell may bedivided into three sectors, where each sector covers 120 degrees. A basestation may provide coverage for one or more sectors. Each symbol of thesynchronization subframe may be associated with a different range indirection/angle. For example, the 14 symbols may collectively cover 120degrees (one sector). In one example, when there are 14 symbols (thus 14direction ranges) per subframe and there are 4 antenna ports, the basestation may transmit beams in 56 (14×4) different directions. In anotherexample, the symbols within a subframe may cover the angular range morethan once. In such an example, if there are 14 symbols within asubframe, the first seven symbols may cover 120 degrees, and then thenext seven symbols may cover the same 120 degrees.

FIG. 6 is an example diagram 600 illustrating resource block usagewithin one symbol of a subframe for millimeter wave communication. Afirst set of RBs (612) may be used to carry BRSs and PBCHs, a second setof RBs (614) may be used to carry an SSS, a PSS, and an ESS, and a thirdset of RBs (616) may be used to carry BRSs and PBCHs. For example, eachRB in the first set of RBs (612) and the third set of RBs (616) may have12 subcarriers, as shown in an example diagram 650. In particular, asshown in the example diagram 650, each RB may include 12 subcarriers,where BRS subcarriers 652 are used to transmit BRSs and PBCH subcarriers654 are used to transmit a PBCH.

When the UE receives the BRS from the base station, the UE may performchannel estimation based on the BRS, where the channel estimate is usedto decode the PBCH. The UE may also use the BRS to perform widebandchannel estimation for each beam and/or to perform narrowband channelestimation for each beam (e.g., when the beam is used for mmWcommunication).

The base station may transmit the BRS using each of multiple antennaports of the base station, by separate subcarriers that arefrequency-division multiplexed for multiple antenna ports and/or bysubcarriers carrying code-division multiplexed information for multipleantenna ports. Each BRS may have the same structure, and thus the BRSsfor all beams may have the same structure. For example, the BRS may be apseudo-random sequence initialized with a cell-specific number. Thus, toenable the UE to determine which BRS corresponds to which beam,subcarriers carrying the BRSs from different beams from respectiveantenna ports may be frequency-division multiplexed or code-divisionmultiplexed (e.g., using orthogonal cover codes). In an aspect, each BRSmay correspond to a respective antenna port. In particular, if thedisjoint subcarriers are frequency-division multiplexed for multipleantenna ports, each of the BRS subcarriers corresponds to a respectiveantenna port. For example, information on eight antenna ports (antennaports #0 -#7) may be frequency-division multiplexed onto 8 subcarriers(e.g., by the base station) as BRS subcarriers in an RB. In such anexample, the remaining four subcarriers in the RB may be used totransmit a PBCH on the PBCH subcarriers. If the code-divisionmultiplexed information is used for multiple antenna ports, informationon the BRS antenna ports are code-division multiplexed on all BRSsubcarriers. For example, information on eight antenna ports (antennaports #0-#7) may be code-division multiplexed onto all 9 BRSsubcarriers, e.g., based on either a Hadamard matrix or a discreteFourier transform (DFT) matrix of length 8. In another aspect, BRSs maybe different in structure for different beams within a symbol. In suchan aspect, the BRS may be a pseudo-random sequence initialized with acombination of a cell-specific number and a beam-specific number.

When the UE receives different beams from different antenna ports of thebase station per symbol, the UE may perform channel estimation (e.g.,narrowband channel estimation) on the received beams based on the BRSscorresponding to the received beams, where each received beamcorresponds to a respective BRS, and the channel estimation is performedon each beam. For example, because the beams for the mmW communicationare directional, some beams may not align with the UE, and thus thenarrowband channel measurement for the BRSs corresponding to such beamsmay be low. On the other hand, narrowband channel measurement for a BRScorresponding to a beam that align with the UE may be high. The channelestimation may be based on measurement of at least one of one of asignal-to-noise ratio, an antenna gain, or a reference signalmeasurement (e.g., reference signal receive power and/or referencesignal received quality) of each received beam, based on a correspondingBRS of the received beam. For example, the UE may rank the receivedbeams based on the narrowband channel estimation of each received beam,where the received beams are ranked in an order of the narrow bandchannel measurements, and may select one or more beams that have thehighest narrowband channel measurements from the received beams based onthe ranking. The beams with the highest narrowband channel measurementsmay be the beams whose narrowband channel measurements are greater thana threshold channel measurement value. In an aspect, when the UEreceives different beams for different symbols, the UE determines thebest beam (e.g., beam with the high narrowband channel measurement)received in each symbol. Thus, for example, if there are 14 symbols, theUE may determine the best beam for each symbol, and thus may determine14 best beams, each best beam corresponding to a respective symbol ofthe 14 symbols. Subsequently, the UE may select one or more beams fromthe best beams, each best beam corresponding to a respective symbol, andtransmit information about the selected one or more beams to the basestation via a feedback signal to the base station. The UE may alsoselect one or more frequency bands that provide the high narrowbandchannel measurements. Thus, in one aspect, the UE may send, to the basestation, a feedback signal including information about N beams and the Mfrequency bands (e.g., M RBs) for each of N beams that provide thehighest channel measurements. For example, referring back to the exampleof FIG. 5, M may range from 1 to 82, as there are 82 RBs carrying theBRSs, and N may range from 1 to 56 (14×4) if there are 14 symbols and 4antenna ports. In an aspect, the feedback signal may further includechannel estimation of the N beams.

The base station may provide the UE with the number of beams whoseinformation should be fed back via the feedback signal. For example, thebase station may indicate to the UE that information about N best beamsshould be fed back to the base station. In an aspect, the base stationmay send the number of beams to the UE via RRC signaling or viainformation carried on a PDCCH. For example, the base station may informthe UE that information about N beams out of the received beams shouldbe fed back to the base station. The UE may transmit the feedback signalto the base station via at least one of a PUCCH and/or in UCI conveyedvia a PUSCH. The number of beams whose information should be fed backvia the feedback signal may depend on whether the UE feeds back theinformation via a PUSCH or via a PUCCH. FIG. 7A is an example diagram700 illustrating a subframe structure when a feedback signal is sent viaa PUCCH. FIG. 7B is an example diagram 750 illustrating a subframestructure when a feedback signal is sent via a PUSCH. As illustrated inFIG. 7A, 12 RBs may be utilized to feedback the information via thePUCCH. As illustrated in FIG. 7B, 72 RBs may be utilized to feedback theinformation via the PUSCH carrying UCI. Because a different amount ofresources are used depending on whether the UE uses the PUCCH or thePUSCH carrying UCI, a number of beams whose information should be fedback is different in FIG. 7A utilizing the PUCCH and FIG. 7B utilizingthe PUSCH carrying UCI. In particular, the UE may send channelinformation for a higher number of beams using the subframe of FIG. 7B(via the PUSCH) than when using the subframe of FIG. 7A (via the PUCCH).For example, the number of beams whose information should be fed backmay be 1 if the UE utilizes the PUCCH to transmit the feedback signal,and the number of beams whose information should be fed back may be 2 ifthe UE utilizes the PUSCH to transmit the feedback signal.

When the UE receives signals via different beams from different antennaports of the base station (e.g., on a per symbol basis), the UE may alsoperform wideband channel estimation on each received beam of eachsymbol. To obtain the wideband channel estimation for a beam, the UE mayobtain a wideband channel estimation measurement for the entirefrequency region of a component carrier per symbol. For example, eachcomponent carrier in the example of FIG. 5 has 100 RBs. A number ofwideband channel estimation measurements per symbol may be equal to thenumber of antenna ports. For example, if the synchronization subframehas 14 symbols as shown in FIG. 5 and if there are 4 antenna ports, thenumber of wideband channel measurements may be 4×14=56.

In an aspect, when the narrowband channel estimation is used with thewideband channel estimation, the wideband channel estimation may be usedto select a beam. For example, a wideband channel estimation of eachbeam may include wideband channel estimation over frequency bands withineach beam. For example, the UE may perform the wideband channelestimation for each beam if the wideband channel estimation for a firstbeam is high and the wideband channel estimation for second, third, andfourth beams are low, the UE may select the first beam to includeinformation about the first beam in the feedback signal. Then, withinthe selected beam, the UE may perform narrowband channel estimation todetermine the best frequency band(s) (e.g., RBs).

In another scenario, the UE may obtain narrowband band channelestimation of each RB of each beam to find the best beam. During theprocess of finding the best beam, UE first determines the number of RBsthat will be used for DL scheduling and/or UL scheduling. The UE maydetermine the number of RBs used for DL scheduling based on path lossfor DL scheduling. The UE may determine the number of RBs used for ULscheduling based on path loss, transmit power, and a buffer size for ULscheduling. After determining the number of RBs (e.g., M RBs), the UEmay find the beam that provides the highest SNR in the best set of MRBs.

When the base station receives the feedback signal, the base station mayselect a beam out of the beams indicated in the feedback signal, andschedule DL communication with the UE based on the selected beam. Forexample, when the base station receives a feedback signal includinginformation about N best beams, the base station may select a beam fromthe N best beams, such that the selected beam may be used forcommunication with the UE. The base station may select the beam out ofthe beams indicated in the feedback signal based on the narrowbandchannel measurement of each of the beams indicated in the feedbacksignal. In an aspect, the base station may select a beam with a highnarrowband measurement. The base station may further consider otherfactors such as interference and noise when selecting the beam. Inaddition, if reciprocity holds between the DL and the UL (e.g., the samechannel may be used for both DL and UL), the channel estimation based onthe BRSs may be used for UL scheduling. In such a case, when the UEsends a feedback signal, the base station may select a beam out of thebeams indicated in the feedback signal to schedule UL communication fromthe UE (e.g., frequency dependent UL scheduling). The beams indicated bythe feedback signal may be the best M bands and N beams, as discussedabove.

In one aspect, a beam refinement reference signal (BRRS) may be utilizedto improve the channel estimation and the beam selection process. A basestation may desire to cover as many directions as possible in a beamselection process. If the base station utilizes beams to cover an entireregion (all possible angles), a total number of beams utilized by thebase station may be so high that transmission of all the beams to coverall directions may be time consuming. Thus, the base station may utilizea limited number of beams sufficient for channel estimation. Inparticular, a total number of beams may be reduced for initialtransmission by the base station. When the UE receives the reducednumber of beams and corresponding BRSs, the UE may perform channelestimation based on the BRSs, and initially select a beam that providesthe optimal performance for the UE (e.g., the best signal condition).When the UE informs the base station of the initially selected beam, thebase station performs transmission using the initially selected beam andone or more other beams that are slightly different in angle from theinitially selected beam. The UE may request the base station to transmitUE-specific BRRSs. When the base station performs transmission using theinitially selected beam and the one or more other beams, the basestation transmits BRRSs corresponding to the initially selected beam andthe one or more other beams. For example, the base station transmits acorresponding BRRS via a respective beam of the initially selected beamand the one or more other beams. Subsequently, the UE performs channelestimation based on the BRRSs corresponding to the initially selectedbeam and the one or more other beams, and finally selects a beam thatprovides optimal performance (e.g., based on the beam with the highestsignal-to-noise ratio, the beam with the highest antenna gain, or thebeam with a highest reference signal measurement). The UE informs thebase station of the finally selected beam, such that the base stationmay transmit using the finally selected beam.

FIGS. 8A-8D are example diagrams 800, 830, 850, and 870 illustrating aprocess of channel estimation based on BRSs and BRRSs, according to anaspect of the disclosure. According to FIG. 8A, a base station 802 inthis example has eight antenna ports, and thus may transmit eight beams811, 812, 813, 814, 815, 816, 817, and 818 in eight different directionsin a symbol. However, utilizing all eight beams may be unnecessarilytime consuming for the base station. Thus, as illustrated in FIG. 8B,the base station 802 may initially utilize every other beam, thusperforming transmission with four beams. In particular, the base station802 initially utilizes the first beam 811, the third beam 813, the fifthbeam 815 and the seventh beam 817, each beam including a correspondingBRS. The base station 802 may transmit signals using the beams in asynchronization subframe. When the UE receives the beams, the UEperforms channel estimation for each beam based on the correspondingBRS, and initially selects a beam with the optimal channel estimationmeasurement. FIG. 8C illustrates that the UE initially selects the fifthbeam 815 based on the channel estimation. The UE informs the basestation of an identifier of the initially selected beam, which is thefifth beam 815. The UE may also request the base station to transmitBRRSs (e.g., UE-specific BRRSs). FIG. 8D illustrates that the basestation 802 utilizes the initially selected beam (the fifth beam 815),and immediately adjacent beams (the fourth beam 814 and the sixth beam816), to transmit corresponding BRRSs. The UE performs channelestimation of the fourth beam 814, the fifth beam 815, and the sixthbeam 816 based on the corresponding BRRSs, and finally selects a beamwith an optimal channel estimation measurement among the fourth beam814, the fifth beam 815, and the sixth beam 816. The UE then informs thebase station 802 of the finally selected beam.

FIG. 9 is an example diagram 900 illustrating a subframe structure fortransmitting a BRRS. In one subframe, the first two symbols may be usedto transmit a PDCCH, and the next nine symbols may be used to transmitPDSCH. BRRSs may be transmitted using the last three symbols in thesubframe. In particular, for example, in the 12th symbol 952, the fourthbeam 814 including a corresponding BRRS may be transmitted. In the 13thsymbol 954, the fifth beam 815 including a corresponding BRRS may betransmitted. In the 14th symbol 956, the sixth beam 816 including acorresponding BRRS may be transmitted. For the 12th symbol 952, the 13thsymbol 954, and the 14th symbol 956, the base station 802 occupies oneRB of every four RBs for transmission of the beams. As a result, theBRRS signal transmitted in each of the 12th, 13th, and 14th symbols maybe repeated three times within the symbol. This allows the UE to trythree different receive combiners or subarrays for each beam within thesymbol. Thus, after three symbols, the UE may determine the besttransmit beam and receive beam pair.

In an aspect, the UE may receive CSI-RSs from the base station, andperform channel estimation (e.g., narrowband channel estimation) forantenna ports of the base station based on the CSI-RSs. FIG. 10 is anexample diagram 1000 illustrating a subframe structure for transmittingCSI-RSs. As illustrated in FIG. 10, within a subframe, the last twosymbols may be dedicated to transmitting beams including CSI-RSs fordifferent antenna ports of the base station. In the example diagram 1000of FIG. 10, the base station has 12 different antenna ports. Thus, theUE may receive CSI-RSs through 12 different beams from the 12 antennaports, and perform channel estimation on the beams corresponding to the12 antenna ports based on the CSI-RSs.

In one aspect, the feedback signal may further include one or morecandidate UL precoders. The UE may select the one or more candidate ULprecoders from a predefined codebook. In one example, the UL precodersmay correspond to various beams of the base station. As discussed above,the UE may perform channel estimation on various beams received from thebase station. The UE may then select the one or more candidate ULprecoders based on channel estimation of various beams corresponding tothe precoders in the codebook. For example, the UE may select one ormore candidate UL precoders that correspond to beams with high channelestimation measurements. The UE transmits the one or more candidate ULprecoders to the base station, e.g., via the feedback signal. The basestation may select a final UL precoder from the one or more candidate ULprecoders, such that the base station may schedule a PUSCH for the UEbased on the selected final UL precoder. The base station may select afinal UL precoder from the one or more candidate UL precoders based onthe channel estimation of beams corresponding to the one or morecandidate UL precoders.

FIG. 11 is an example diagram 1100 illustrating communication between auser equipment and a base station in millimeter wave communication,according to an aspect of the disclosure. The example diagram 1100involves communication between a UE 1102 and a base station 1104. In theexample diagram 1100, the base station has four antenna ports, andtransmits four beams per symbol. At 1112, the base station 1104transmits four beams including corresponding signals (BRSs or BRRSs orCSI-RSs) in a first angular range corresponding to the first symbol(symbol 0). At 1114, the base station 1104 transmits four beamsincluding corresponding signals (BRSs or BRRSs or CSI-RSs) in a secondangular range corresponding to the second symbol (symbol 1). The basestation 1104 continues to transmit beams including corresponding signals(BRSs or BRRSs or CSI-RSs) in different angular ranges for differentsymbols. At 1118, the base station 1104 transmits four beams includingcorresponding signals (BRSs or BRRSs or CSI-RSs) in a fourteenth angularrange corresponding to the fourteenth symbol (symbol 13). Thus, in thisexample, the base station 1104 transmits 4 beams in different directionsper symbol, and thus transmits 56 beams in various directions over 14symbols. At 1120, the base station 1104 may send a number of beams whoseinformation should be fed back to the base station, where the number ofbeams may be N. In an aspect, at 1120, the base station 1104 sends thenumber of beams to the UE 1102 via RRC signaling or via informationconveyed via a PDCCH. In an aspect, bits in DCI transmitted to the UE1102 may be reserved to carry information about the number of beams N.

At 1122, the UE may perform wideband channel estimation and/ornarrowband channel estimation for each received beam carrying acorresponding signal (e.g., BRS or BRRSs or CSI-RS), and determinesdesirable frequency bands and beams based on the wideband channelestimation of each received beam and/or narrowband channel estimation ofeach received beam. For example, the UE may perform wideband channelestimation of each beam to determine an SNR for the entire frequencyregion for each beam in each symbol, and then may perform narrowbandchannel estimation to determine SNR values for each RB used to carry thecorresponding signal (BRS or BRRSs or CSI-RS) in each symbol. In oneexample, the UE may select one or more beams based on the widebandchannel estimation, and then may select one or more RBs based on thenarrowband channel estimation. For example, the UE may determine Mfrequency bands (e.g., M RBs) for each of N beams that correspond to thebest narrowband channel measurements. As discussed above, N may be lessthan or equal to a number of symbols times a number of antenna ports,and M may be less than or equal to the number of RBs carrying thesignals (BRSs or BRRSs or CSI-RSs) in one symbol, where a size of eachfrequency band may correspond to a size of each RB in frequency. At1124, the UE sends feedback information to the base station 1104, wherethe feedback information may include information on the M frequencybands for each of N beams that correspond to the best narrowband channelmeasurements. In an aspect, the feedback information may be transmittedto the base station 1104 via at least one of a PUCCH and/or in UCIconveyed via a PUSCH. In an aspect, the feedback information may betransmitted to the base station 1104 through a RACH subframe. At 1126,the base station 1104 may select a beam among the N beams. Further, forthe selected beam, the base station 1104 may also select a frequencyband among M frequency bands, to schedule downlink (DL) communicationwith the UE 1102. At 1128, the base station 1104 performs DLcommunication with the UE 1102 via the beam selected among the N beams.

FIG. 12 is a flowchart 1200 of a method of wireless communication. Themethod may be performed by a UE (e.g., the UE 1102, the apparatus1402/1402′). At 1202, the UE receives, from a base station, a pluralityof signals through a plurality of beams of the base station, each of theplurality of beams corresponding to a respective antenna port of aplurality of antenna ports of the base station. In an aspect, theplurality of signals may include a plurality of beam reference signals,a plurality of beam refinement reference signals, a plurality ofCSI-RSs, or a combination thereof. For example, as discussed supra, anantenna port including a set of antennas (e.g., 64 antennas) maytransmit one beam, and multiple antenna ports may transmit multiplebeams respectively, each in a different direction. For example, asillustrated in FIG. 4A, a base station 402 in the example diagram 400has four antenna ports, and may transmit four beams 412, 414, 416, and418 in four different directions in the first symbol. For example, asdiscussed supra, the base station may transmit BRSs in variousdirections via corresponding beams so that the UE may identify the bestbeam of the one or more beams received from the base station based onmeasurements on the BRSs. For example, as discussed supra, when the basestation performs transmission using the initially selected beam and oneor more other beams, the base station transmits BRRSs corresponding tothe initially selected beam and the one or more other beams. In anaspect, the plurality of beam reference signals may be received during asynchronization subframe. For example, as illustrated in FIG. 5, thesynchronization subframe may be used to carry BRSs.

At 1204, the UE may receive from the base station a number of beamswhose information should be fed back to the base station. For example,as discussed supra, the base station may inform the UE that informationabout N beams should be fed back to the base station. In an aspect, thenumber of beams may be based on whether the UE transmits a feedbacksignal via a PUSCH or a PUCCH. In such an aspect, the number of beamswhose information should be fed back is higher for feedback via thePUSCH than for feedback via the PUCCH. For example, as illustrated inFIGS. 7A and 7B, because a different amount of resources are useddepending on whether the UE uses the PUCCH or the PUSCH carrying UCI, anumber of beams whose information should be fed back is different inFIG. 7A that utilizes the PUCCH and FIG. 7B that utilizes the PUSCHcarrying UCI. For example, as illustrated in FIGS. 7A and 7B, the UEsends channel information of a higher number of beams using the subframeof FIG. 7B (via the PUSCH) than using the subframe of FIG. 7A (via thePUCCH). In an aspect, the number of beams may be two. In such an aspect,a strongest beam of the two beams may be used as an active beam for theUE and a weakest beam of the two beams is used as a candidate beam forthe UE. For example, as discussed supra, the number of beams whoseinformation should be fed back may be 2 if the UE utilizes the PUSCH totransmit the feedback signal. For example, as discussed supra, the basestation may select a beam out of the beams indicated in the feedbacksignal, such that the UE may utilize the selected beam as an activebeam.

At 1206, the UE performs channel estimation for each beam of theplurality of beams from the plurality of antenna ports based on theplurality of signals. For example, as discussed supra, when the UEreceives different beams from different antenna ports of the basestation per symbol, the UE may perform narrowband channel estimation onreceived beams based on the BRSs corresponding to the received beams. Inan aspect, the channel estimation may include at least one of narrowbandchannel estimation or wideband channel estimation. For example, asdiscussed supra, the UE may also use the BRS to perform wideband channelestimation for each beam and/or to perform narrowband channel estimationfor each beam. In an aspect, the channel estimation is based on ameasurement of at least one of a signal-to-noise ratio, an antenna gain,or a reference signal measurement of each of the plurality of beams. Forexample, as discussed supra, the measurement of the channel estimationmay be based on at least one of one of a signal-to-noise ratio, anantenna gain, or a reference signal measurement (e.g., reference signalreceive power and/or reference signal received quality) of the receivedbeams, based on the BRSs. In an aspect, the plurality of beams from theplurality of antenna ports are received at different directions.

At 1210, the UE may perform additional features as discussed supra. At1212, in an aspect, the UE may select one or more candidate uplinkprecoders from a predefined codebook. For example, as discussed supra,the UE may select the one or more candidate UL precoders based onchannel estimation of various beams that corresponds to the precoders inthe codebook.

At 1214, the UE selects one or more beams from the plurality of beamsbased on the channel estimation. In an aspect, the UE may select the oneor more beams among the plurality of beams by selecting a beam with ahigh measurement of the channel estimation for each of a plurality ofsymbols, the high measurement being greater than a threshold measurementfor the channel estimation, each symbol being associated with acorresponding set of beams from the plurality of antenna ports, wherethe one or more beams are selected among the beams with the highmeasurements for the plurality of symbols. For example, as discussedsupra, the UE selects one or more beams that have the high narrowbandchannel measurements based on the ranking, where the beams with the highnarrowband channel measurements may be the beams whose narrowbandchannel measurements are greater than a threshold channel measurementvalue. For example, as discussed supra, when the UE receives differentbeams for different symbols, the UE determines the best beam (e.g., beamwith the high narrowband channel measurement) received in each symbol,and subsequently may select one or more beams from the best beams, eachbest beam corresponding to a respective symbol. In an aspect, the one ormore beams are selected within one or more frequency bands based on thechannel estimation. For example, as discussed supra, the UE may alsoselect one or more frequency bands that provide the high narrowbandchannel measurement.

At 1216, the UE transmits, to the base station, a feedback signalincluding information about one or more beams selected from theplurality of beams within one or more frequency bands. For example, asdiscussed supra, the UE may select one or more beams out of the bestbeams, and transmit information about the selected one or more beams tothe base station via a feedback signal to the base station, and may alsoselect one or more frequency bands that provide the high narrowbandchannel measurement. For example, as discussed supra, the UE may send,to the base station, a feedback signal including information about thebest M bands (e.g., M RBs) and N beams. In an aspect, a number of theone or more beams may be based on the number of beams whose informationshould be fed back to the base station (e.g., where the number of beamswhose information should be fed back to the base station is receivedfrom the base station at 1204). For example, as discussed supra, thebase station may send the UE the number of beams whose informationshould be fed back to the base station. In an aspect, the feedbacksignal is transmitted to the base station via at least one of a PUCCH orin UCI conveyed via a PUSCH. For example, as illustrated in FIGS. 7A and7B, the UE may transmit the feedback signal via at least one of a PUCCHor in UCI conveyed via a PUSCH. In an aspect, the feedback signal istransmitted to the base station through a RACH subframe.

In an aspect, the feedback signal may further include the one or morecandidate uplink precoders selected by the UE. In such an aspect, theone or more candidate uplink precoders may each be a candidate for aprecoder used for scheduling a PUSCH. For example, as discussed supra,the feedback signal may further include one or more candidate ULprecoders. For example, as discussed supra, the base station may selecta final UL precoder from the one or more candidate UL precoders, suchthat the base station may schedule a PUSCH for the UE based on the finalUL precoder.

FIG. 13 is a flowchart 1300 of a method of wireless communication,expanding from the flowchart 1200 of FIG. 12, according to an aspect.The method may be performed by a UE (e.g., the UE 1102, the apparatus1402/1402′). The features of the flowchart 1300 may continue from 1208of FIG. 12. At 1208, the UE receives a plurality of ESSs, each ESSindicating a corresponding symbol of a plurality of symbols. At 1210,the UE associates each set of beams to a respective symbol based on acorresponding ESS. For example, as discussed supra, the UE may receivean ESS, where the ESS may be used to indicate a symbol, in order toenable the UE to identify a particular symbol index within the subframe.For example, as discussed supra, for each received beam at the UE, theUE may identify the received beam based on a BRS received from the basestation via the received beam, and may identify a symbol for thereceived beam based on an ESS received via the received beam.

At 1306, the UE ranks each beam of the plurality of beams based on thechannel estimation, where the UE may select the one or more beams fromthe plurality of beams based on the ranking (e.g., at 1214). Forexample, as discussed supra, the UE may rank the beams based on thenarrowband channel estimation of each beam, and select one or more beamsthat have the high narrowband channel measurements based on the ranking.In an aspect, the measuring of the channel estimation for the pluralityof beams at 1206 may include performing wideband channel estimation foreach beam of the plurality of beams, where the ranking is further basedon the wideband channel estimation for the plurality of beams. Forexample, as discussed supra, if the wideband channel estimation for afirst beam is high and the wideband channel estimation for second,third, and fourth beams are low, the UE may select the first beam toinclude information about the first beam in the feedback signal.

FIG. 14 is a conceptual data flow diagram 1400 illustrating the dataflow between different means/components in an exemplary apparatus 1402.The apparatus may be a UE. The apparatus includes a reception component1404, a transmission component 1406, a signal management component 1408,a channel estimation component 1410, an ESS processing component 1412, aselection management component 1414, and a feedback management component1416.

The signal management component 1408 receives, from a base station(e.g., base station 1430), via the reception component 1404 at 1452 and1454, a plurality of signals through a plurality of beams of the basestation, each of the plurality of beams corresponding to a respectiveantenna port of a plurality of antenna ports of the base station. In anaspect, the plurality of signals may include a plurality of beamreference signals, a plurality of beam refinement reference signals, aplurality of CSI-RSs, or a combination thereof. In an aspect, theplurality of signals may be received during a synchronization subframe.In an aspect, the selection management component 1414 may receive, viathe reception component 1404, at 1452 and 1470, from the base station anumber of beams whose information should be fed back to the basestation. In an aspect, the number of beams may be based on whether theUE transmits a feedback signal via physical uplink shared channel orphysical uplink control channels. In such an aspect, the number of beamswhose information should be fed back is higher for feedback via thePUSCH than for feedback via the PUCCH. In an aspect, the number of beamsmay be two. In such an aspect, a strongest beam of the two beams may beused as an active beam for the UE and a weakest beam of the two beams isused as a candidate beam for the UE. The signal management component maycommunicate information about beams and corresponding signals (e.g.,BRSs and/or BRRSs) to the channel estimation component 1410 at 1456.

The channel estimation component 1410 performs channel estimation foreach beam of the plurality of beams from the plurality of antenna portsbased on the plurality of signals. In an aspect, the channel estimationmay include at least one of narrowband channel estimation or widebandchannel estimation. In an aspect, the channel estimation is based on ameasurement of at least one of a signal-to-noise ratio, an antenna gain,or a reference signal measurement of each of the plurality of beams. Inan aspect, the plurality of beams from the plurality of antenna portsare directed at different directions. The channel estimation component1410 may provide results of the measurements of the channel estimationto the selection management component 1414 at 1458.

The ESS processing component 1412 receives, via the reception component1404 at 1452 and 1460, a plurality of ESSs, each ESS indicating acorresponding symbol of the plurality of symbols. The ESS processingcomponent 1412 associates each set of beams of the plurality of beams toa respective symbol of the plurality of symbols based on a correspondingESS of the plurality of ESSs. The ESS processing component 1412 mayprovide the association information of each set of beams to a respectivesymbol to the selection management component 1414, at 1462.

The selection management component 1414 selects one or more beams fromthe plurality of beams based on the channel estimation. The selectionmanagement component 1414 ranks the plurality of beams based on thechannel estimation. In an aspect, the ranking may be further based onwideband channel estimation for the plurality of beams. In such anaspect, the selection management component 1414 may select the one ormore beams from the plurality of beams based on the ranking. In anaspect, the channel estimation component 1410 may perform widebandchannel estimation for each beam of the plurality of beams, where theranking by the selection management component 1414 is further based onwideband channel estimation for the plurality of beams. In an aspect,the UE may select the one or more beams among the plurality of beams byselecting a beam with a high measurement of the narrowband channelestimation for each of a plurality of symbols, the high measurementbeing greater than a threshold measurement for the narrowband channelestimation, each symbol being associated with a corresponding set ofbeams from the plurality of antenna ports, where the one or more beamsare selected among the beams with the high measurements for theplurality of symbols. In an aspect, the one or more beams are selectedwithin one or more frequency bands based on the measured narrowbandchannel estimation. The selection management component 1414 may provideinformation about the one or more beams selected from the plurality ofbeams to the feedback management component 1416 at 1464.

The feedback management component 1416 transmits, to the base station,via the transmission component 1406 at 1466 and 1468, a feedback signalincluding information about the one or more beams selected from theplurality of beams within one or more frequency bands. In an aspect, anumber of the one or more beams may be based on the number of beamswhose information should be fed back to the base station. In an aspect,the feedback signal may be transmitted to the base station via at leastone of a PUCCH or in UCI conveyed via a PUSCH. In an aspect, thefeedback signal may be transmitted to the base station through a RACHsubframe. In an aspect, the selection management component 1414 mayselect one or more candidate uplink precoders from a predefinedcodebook, where the feedback signal further includes the one or morecandidate uplink precoders. In such an aspect, the one or more candidateuplink precoders may be selected based on the channel estimation. Insuch an aspect, the one or more candidate uplink precoders may each be acandidate for a precoder used for scheduling a PUSCH.

The apparatus may include additional components that perform each of theblocks of the algorithm in the aforementioned flowcharts of FIGS. 12 and13. As such, each block in the aforementioned flowcharts of FIGS. 8 and9 may be performed by a component and the apparatus may include one ormore of those components. The components may be one or more hardwarecomponents specifically configured to carry out the statedprocesses/algorithm, implemented by a processor configured to performthe stated processes/algorithm, stored within a computer-readable mediumfor implementation by a processor, or some combination thereof.

FIG. 15 is a diagram 1500 illustrating an example of a hardwareimplementation for an apparatus 1402′ employing a processing system1514. The processing system 1514 may be implemented with a busarchitecture, represented generally by the bus 1524. The bus 1524 mayinclude any number of interconnecting buses and bridges depending on thespecific application of the processing system 1514 and the overalldesign constraints. The bus 1524 links together various circuitsincluding one or more processors and/or hardware components, representedby the processor 1504, the components 1404, 1406, 1408, 1410, 1412,1414, 1416, and the computer-readable medium/memory 1506. The bus 1524may also link various other circuits such as timing sources,peripherals, voltage regulators, and power management circuits, whichare well known in the art, and therefore, will not be described anyfurther.

The processing system 1514 may be coupled to a transceiver 1510. Thetransceiver 1510 is coupled to one or more antennas 1520. Thetransceiver 1510 provides a means for communicating with various otherapparatus over a transmission medium. The transceiver 1510 receives asignal from the one or more antennas 1520, extracts information from thereceived signal, and provides the extracted information to theprocessing system 1514, specifically the reception component 1404. Inaddition, the transceiver 1510 receives information from the processingsystem 1514, specifically the transmission component 1406, and based onthe received information, generates a signal to be applied to the one ormore antennas 1520. The processing system 1514 includes a processor 1504coupled to a computer-readable medium/memory 1506. The processor 1504 isresponsible for general processing, including the execution of softwarestored on the computer-readable medium/memory 1506. The software, whenexecuted by the processor 1504, causes the processing system 1514 toperform the various functions described supra for any particularapparatus. The computer-readable medium/memory 1506 may also be used forstoring data that is manipulated by the processor 1504 when executingsoftware. The processing system 1514 further includes at least one ofthe components 1404, 1406, 1408, 1410, 1412, 1414, 1416. The componentsmay be software components running in the processor 1504,resident/stored in the computer readable medium/memory 1506, one or morehardware components coupled to the processor 1504, or some combinationthereof. The processing system 1514 may be a component of the UE 350 andmay include the memory 360 and/or at least one of the TX processor 368,the RX processor 356, and the controller/processor 359.

In one configuration, the apparatus 1402/1402′ for wirelesscommunication includes means for receiving, from a base station, aplurality of signals through a plurality of beams of the base station,each of the plurality of beams corresponding to a respective antennaport of a plurality of antenna ports of the base station, and means forperforming channel estimation for each beam of the plurality of beamsfrom the plurality of antenna ports based on the plurality of signals.In an aspect, the apparatus 1402/1402′ includes means for selecting oneor more beams from the plurality of beams based on the channelestimation, and means for transmitting, to the base station, a feedbacksignal including information about the one or more beams selected fromthe plurality of beams within one or more frequency bands. In an aspect,the apparatus 1402/1402′ includes means for ranking the plurality ofbeams based on the channel estimation, where the means for selecting theone or more beams from the plurality of beams is based on the ranking.In an aspect, the means for measuring the channel estimation for theplurality of beams may be configured to perform wideband channelestimation for each beam of the plurality of beams, where the means forranking is configured to rank the plurality of beams further based onthe wideband channel estimation for the plurality of beams. In anaspect, the means for selecting the one or more beams among theplurality of beams is configured to: select a beam with a highmeasurement of the channel estimation for each of a plurality ofsymbols, the high measurement being greater than a threshold measurementfor the channel estimation, each symbol being associated with acorresponding set of beams from the plurality of antenna ports, whereinthe one or more beams are selected among the beams with the highmeasurements for the plurality of symbols.

In an aspect, the apparatus 1402/1402′ includes means for receiving aplurality of ESSs, each ESS indicating a corresponding symbol of theplurality of symbols, and means for associating each set of beams of theplurality of beams to a respective symbol of the plurality of symbolsbased on a corresponding ESS of the plurality of ESSs. In an aspect, theapparatus 1402/1402′ includes means for selecting one or more candidateuplink precoders from a predefined codebook, where the feedback signalfurther includes the one or more candidate uplink precoders. In anaspect, the apparatus 1402/1402′ includes means for receiving from thebase station a number of beams whose information should be fed back tothe base station.

The aforementioned means may be one or more of the aforementionedcomponents of the apparatus 1402 and/or the processing system 1514 ofthe apparatus 1402′ configured to perform the functions recited by theaforementioned means. As described supra, the processing system 1514 mayinclude the TX Processor 368, the RX Processor 356, and thecontroller/processor 359. As such, in one configuration, theaforementioned means may be the TX Processor 368, the RX Processor 356,and the controller/processor 359 configured to perform the functionsrecited by the aforementioned means.

FIG. 16 is a flowchart 1600 of a method of wireless communication. Themethod may be performed by a base station (e.g., the base station 1104 ,the apparatus 1302/1302′). At 1602, the base station may transmit aplurality of signals through a plurality of beams of the base station,each of the plurality of beams corresponding to a respective antennaport of a plurality of antenna ports of the base station. In an aspect,the plurality of signals include a plurality of beam reference signals,a plurality of beam refinement reference signals, a plurality ofCSI-RSs, or a combination thereof. For example, as discussed supra, anantenna port including a set of antennas (e.g., 64 antennas) maytransmit one beam, and multiple antenna ports may transmit multiplebeams respectively, each in a different direction. For example, asillustrated in FIG. 4A, a base station 402 in the example diagram 400has four antenna ports, and may transmit four beams 412, 414, 416, and418 in four different directions in the first symbol. For example, asdiscussed supra, the base station may transmit BRSs in variousdirections via corresponding beams so that the UE may identify the bestbeam of the one or more beams received from the base station based onmeasurements on the BRSs. For example, as discussed supra, when the basestation performs transmission using the initially selected beam and oneor more other beams, the base station transmits BRRSs corresponding tothe initially selected beam and the one or more other beams. In anaspect, the plurality of beam reference signals are transmitted during asynchronization subframe. For example, as illustrated in FIG. 5, thesynchronization subframe may be used to carry BRSs.

In an aspect, one or more beams may be selected based on channelestimation for each beam of the plurality of beams and the channelestimation includes at least one of narrowband channel estimationwideband channel estimation. For example, as discussed supra, the UE mayalso use the BRS to perform wideband channel estimation for each beamand/or to perform narrowband channel estimation for each beam. In anaspect, the base station transmits the plurality of signals by sweepingthrough a plurality of directions in different symbols to transmit theplurality of signals. For example, as discussed supra, the base stationmay use multiple antenna ports in a cell specific manner in a firstsymbol of a synchronization sub-frame to sweep in multiple directions,and then may sweep in multiple directions using the multiple antennaports in a cell specific manner in another symbol of the synchronizationsubframe. For example, as illustrated in FIG. 11, the base station maytransmit beams at four different directions per symbol, over 14 symbols.

At 1604, the base station informs a UE of a number of beams whoseinformation should be fed back to the base station from the UE. Forexample, as discussed supra, the base station may inform the UE thatinformation about N beams should be fed back to the base station. In anaspect, the base station may inform the UE via RRC signaling or viainformation conveyed over a PDCCH. For example, as discussed supra, thebase station sends the number of beams to the UE through RRC signalingor a PDCCH. In an aspect, one or more bits are reserved in DCItransmitted to the UE to inform the UE of the number of beams whoseinformation should be fed back to the base station. For example, asillustrated in FIG. 11, bits in DCI transmitted to the UE 1102 may bereserved to carry information about the number of beams. In an aspect,the number of beams is determined based on whether the feedback signalis received via a PUSCH or via a PUCCH. For example, as illustrated inFIGS. 7A and 7B, because a different amount of resources are useddepending on whether the UE uses the PUCCH or the PUSCH carrying UCI, anumber of beams whose information should be fed back is different inFIG. 7A that utilizes the PUCCH and FIG. 7B that utilizes the PUSCHcarrying UCI. For example, as illustrated in FIGS. 7A and 7B, the UEsends channel information of a higher number of beams using the subframeof FIG. 7B (via the PUSCH) than using the subframe of FIG. 7A (via thePUCCH). In such an aspect, the number of beams whose information shouldbe fed back is higher for feedback via the PUSCH than for feedback viathe PUCCH. In an aspect, the number of beams is two. In such an aspect,a strongest beam of the two beams is used as an active beam for the UEand a weakest beam of the two beams is used as a candidate beam for theUE. For example, as discussed supra, the number of beams whoseinformation should be fed back may be 2 if the UE utilizes the PUSCH totransmit the feedback signal. For example, as discussed supra, the basestation may select a beam out of the beams indicated in the feedbacksignal, such that the UE may utilize the selected beam as an activebeam.

At 1606, the base station receives, from the UE, a feedback signalincluding information about one or more beams selected from theplurality of beams within one or more frequency bands. For example, asdiscussed supra, the UE may select one or more beams out of the bestbeams, and transmit information about the selected one or more beams tothe base station via a feedback signal to the base station, and may alsoselect one or more frequency bands that provide the high narrowbandchannel measurement. For example, as discussed supra, the base stationmay receive, from the UE, a feedback signal including information aboutthe best M bands (e.g., M RBs) and N beams. In an aspect, the feedbacksignal is received from the UE via at least one of a PUCCH or UCI in aPUSCH. For example, as illustrated in FIGS. 7A and 7B, the base stationmay receive, from the UE, the feedback signal via at least one of aPUCCH or UCI in a PUSCH. In an aspect, the feedback signal is receivedfrom the UE through a RACH subframe. In an aspect, a number of the oneor more beams may be based on the number of beams whose informationshould be fed back to the base station. For example, as discussed supra,the base station may send the UE the number of beams whose informationshould be fed back to the base station.

At 1608, in an aspect, the base station may schedule resources for theUE on a PUSCH based on the feedback signal. In such an aspect, thefeedback signal may further include one or more candidate uplinkprecoders, and the scheduling the PUSCH may further include: selecting afinal uplink precoder from the one or more candidate uplink precoders,and scheduling the PUSCH based on the final uplink precoder. Forexample, as discussed supra, the base station may select a final ULprecoder from the one or more candidate UL precoders included in thefeedback signal, such that the base station may schedule a PUSCH for theUE based on the final UL precoder. In such an aspect, the one or morecandidate uplink precoders are from a predefined codebook. For example,as discussed supra, the feedback signal may further include one or morecandidate UL precoders.

At 1610, the base station may choose a beam among the one or more beamsbased on the feedback signal. At 1612, the base station performscommunication with the UE based on the feedback signal. In an aspect,the base station performs communication with the UE via the chosen beam.For example, as discussed supra, when the base station receives thefeedback signal, the base station may select a beam out of the beamsindicated in the feedback signal, and schedule DL communication with theUE based on the selected beam. For example, as discussed supra, the basestation may select the beam based on the channel estimation of the beamsindicated in the feedback signal. For example, as discussed supra, thebase station may select the beam out of the beams indicated in thefeedback signal based on the narrowband channel measurements of thebeams.

FIG. 17 is a conceptual data flow diagram 1700 illustrating the dataflow between different means/components in an exemplary apparatus 1702.The apparatus may be a base station. The apparatus includes a receptioncomponent 1704, a transmission component 1706, a signal managementcomponent 1708, a beam number determination component 1710, a feedbackprocessing component 1712, a beam selection component 1714, acommunication management component 1716.

The signal management component 1708 transmits, to a UE (e.g., UE 1730),via the transmission component 1706 at 1752 and 1754, a plurality ofsignals through a plurality of beams of the base station, each of theplurality of beams corresponding to a respective antenna port of aplurality of antenna ports of the base station. In an aspect, theplurality of signals include a plurality of beam reference signal, aplurality of beam refinement reference signals, a plurality of CSI-RSs,or a combination thereof. In an aspect, the plurality of beam referencesignals are transmitted during a synchronization subframe. In an aspect,the one or more beams are selected based on channel estimation for eachbeam of the plurality of beams and the channel estimation includes atleast one of narrowband channel estimation or wideband channelestimation. In an aspect, the signal management component 1708 transmitsthe plurality of signals by sweeping through a plurality of directionsin different symbols to transmit the plurality of signals.

The beam number determination component 1710 informs the UE, via thetransmission component 1706, a number of beams whose information shouldbe fed back to the base station from the UE, at 1756 and 1754. In anaspect, the beam number determination component 1710 informs the UE ofthe number of beams through RRC signaling or a PDCCH. In an aspect, oneor more bits are reserved in DCI transmitted to the UE to inform the UEof the number of beams whose information should be fed back to the basestation. In an aspect, the number of beams is determined based onwhether the feedback signal is received via a PUCCH or a PUSCH. In suchan aspect, In such an aspect, the number of beams whose informationshould be fed back is higher for feedback via the PUSCH than forfeedback via the PUCCH. In an aspect, the number of beams is two. Insuch an aspect, a strongest beam of the two beams is used as an activebeam for the UE and a weakest beam of the two beams is used as acandidate beam for the UE.

The feedback processing component 1712 receives, from the UE, via thereception component 1704 at 1758 and 1760, a feedback signal includinginformation about one or more beams selected from the plurality of beamswithin one or more frequency bands. In an aspect, the feedback signal isreceived from the UE via at least one of a PUCCH or in UCI conveyed viaa PUSCH. In an aspect, the feedback signal is received from the UEthrough a RACH subframe. The feedback processing component 1712 mayforward the feedback signal to the communication management component1716, at 1761, and to the beam selection component 1714, at 1762.

The communication management component 1716 may schedule resources forthe UE on a PUSCH based on the feedback signal. In such an aspect, thefeedback signal may further include one or more candidate uplinkprecoders, and the communication management component 1716 may schedulethe PUSCH by: selecting a final uplink precoder from the one or morecandidate uplink precoders, and scheduling the PUSCH based on the finaluplink precoder. In such an aspect, the one or more candidate uplinkprecoders are from a predefined codebook.

The beam selection component 1714 may choose a beam among the one ormore beams based on the feedback signal. The beam selection component1714 may forward information about the chosen beam to the communicationmanagement component 1716, at 1764. The communication managementcomponent 1716 performs communication with the UE based on the feedbacksignal, via the transmission component 1706, at 1766 and 1754. In anaspect, the communication management component 1716 may performcommunication with the UE via the chosen beam. The communicationmanagement component 1716 may also receive communication from the UE viathe reception component 1704, at 1758 and 1766.

The apparatus may include additional components that perform each of theblocks of the algorithm in the aforementioned flowcharts of FIG. 16. Assuch, each block in the aforementioned flowcharts of FIG. 16 may beperformed by a component and the apparatus may include one or more ofthose components. The components may be one or more hardware componentsspecifically configured to carry out the stated processes/algorithm,implemented by a processor configured to perform the statedprocesses/algorithm, stored within a computer-readable medium forimplementation by a processor, or some combination thereof.

FIG. 18 is a diagram 1800 illustrating an example of a hardwareimplementation for an apparatus 1702′ employing a processing system1814. The processing system 1814 may be implemented with a busarchitecture, represented generally by the bus 1824. The bus 1824 mayinclude any number of interconnecting buses and bridges depending on thespecific application of the processing system 1814 and the overalldesign constraints. The bus 1824 links together various circuitsincluding one or more processors and/or hardware components, representedby the processor 1804, the components 1704, 1706, 1708, 1710, 1712,1714, 1716, and the computer-readable medium/memory 1806. The bus 1824may also link various other circuits such as timing sources,peripherals, voltage regulators, and power management circuits, whichare well known in the art, and therefore, will not be described anyfurther.

The processing system 1814 may be coupled to a transceiver 1810. Thetransceiver 1810 is coupled to one or more antennas 1820. Thetransceiver 1810 provides a means for communicating with various otherapparatus over a transmission medium. The transceiver 1810 receives asignal from the one or more antennas 1820, extracts information from thereceived signal, and provides the extracted information to theprocessing system 1814, specifically the reception component 1704. Inaddition, the transceiver 1810 receives information from the processingsystem 1814, specifically the transmission component 1706, and based onthe received information, generates a signal to be applied to the one ormore antennas 1820. The processing system 1814 includes a processor 1804coupled to a computer-readable medium/memory 1806. The processor 1804 isresponsible for general processing, including the execution of softwarestored on the computer-readable medium/memory 1806. The software, whenexecuted by the processor 1804, causes the processing system 1814 toperform the various functions described supra for any particularapparatus. The computer-readable medium/memory 1806 may also be used forstoring data that is manipulated by the processor 1804 when executingsoftware. The processing system 1814 further includes at least one ofthe components 1704, 1706, 1708, 1710, 1712, 1714, 1716. The componentsmay be software components running in the processor 1804,resident/stored in the computer readable medium/memory 1806, one or morehardware components coupled to the processor 1804, or some combinationthereof. The processing system 1814 may be a component of the eNB 310and may include the memory 376 and/or at least one of the TX processor316, the RX processor 370, and the controller/processor 375.

In one configuration, the apparatus 1702/1702′ for wirelesscommunication includes means for transmitting, to a UE, a plurality ofsignals through a plurality of beams of the base station, each of theplurality of beams corresponding to a respective antenna port of aplurality of antenna ports of the base station, and means for receiving,from the UE, a feedback signal including information about one or morebeams selected from the plurality of beams within one or more frequencybands. In an aspect, the means for transmitting the plurality of signalsis configured to sweep through a plurality of directions in differentsymbols to transmit the plurality of signals. In an aspect, theapparatus 1702/1702′ includes means for scheduling a PUSCH based on thefeedback signal. In an aspect, the feedback signal further includes oneor more candidate uplink precoders, and the means for scheduling thePUSCH is further configured to: select a final uplink precoder from theone or more candidate uplink precoders, and schedule the PUSCH based onthe final uplink precoder. In an aspect, the apparatus 1702/1702′includes means for performing communication with the UE based on thefeedback signal. In an aspect, the apparatus 1702/1702′ includes meansfor choosing a beam among the one or more beams based on the feedbacksignal, where the communication with the UE is performed via the chosenbeam. In an aspect, the apparatus 1702/1702′ includes means forinforming the UE a number of beams whose information should be fed backto the base station from the UE. In an aspect, the means for informingis configured to inform the UE of the number of beams through RRCsignaling or a PDCCH.

The aforementioned means may be one or more of the aforementionedcomponents of the apparatus 1702 and/or the processing system 1814 ofthe apparatus 1702′ configured to perform the functions recited by theaforementioned means. As described supra, the processing system 1814 mayinclude the TX Processor 316, the RX Processor 370, and thecontroller/processor 375. As such, in one configuration, theaforementioned means may be the TX Processor 316, the RX Processor 370,and the controller/processor 375 configured to perform the functionsrecited by the aforementioned means.

It is understood that the specific order or hierarchy of blocks in theprocesses/flowcharts disclosed is an illustration of exemplaryapproaches. Based upon design preferences, it is understood that thespecific order or hierarchy of blocks in the processes/flowcharts may berearranged. Further, some blocks may be combined or omitted. Theaccompanying method claims present elements of the various blocks in asample order, and are not meant to be limited to the specific order orhierarchy presented.

The previous description is provided to enable any person skilled in theart to practice the various aspects described herein. Variousmodifications to these aspects will be readily apparent to those skilledin the art, and the generic principles defined herein may be applied toother aspects. Thus, the claims are not intended to be limited to theaspects shown herein, but is to be accorded the full scope consistentwith the language claims, wherein reference to an element in thesingular is not intended to mean “one and only one” unless specificallyso stated, but rather “one or more.” The word “exemplary” is used hereinto mean “serving as an example, instance, or illustration.” Any aspectdescribed herein as “exemplary” is not necessarily to be construed aspreferred or advantageous over other aspects. Unless specifically statedotherwise, the term “some” refers to one or more. Combinations such as“at least one of A, B, or C,” “one or more of A, B, or C,” “at least oneof A, B, and C,” “one or more of A, B, and C,” and “A, B, C, or anycombination thereof” include any combination of A, B, and/or C, and mayinclude multiples of A, multiples of B, or multiples of C. Specifically,combinations such as “at least one of A, B, or C,” “one or more of A, B,or C,” “at least one of A, B, and C,” “one or more of A, B, and C,” and“A, B, C, or any combination thereof” may be A only, B only, C only, Aand B, A and C, B and C, or A and B and C, where any such combinationsmay contain one or more member or members of A, B, or C. All structuraland functional equivalents to the elements of the various aspectsdescribed throughout this disclosure that are known or later come to beknown to those of ordinary skill in the art are expressly incorporatedherein by reference and are intended to be encompassed by the claims.Moreover, nothing disclosed herein is intended to be dedicated to thepublic regardless of whether such disclosure is explicitly recited inthe claims. The words “module,” “mechanism,” “element,” “device,” andthe like may not be a substitute for the word “means.” As such, no claimelement is to be construed as a means plus function unless the elementis expressly recited using the phrase “means for.”

What is claimed is:
 1. A method of wireless communication by a userequipment (UE), comprising: receiving, from a base station, a pluralityof signals through a plurality of beams of the base station, each of theplurality of beams corresponding to a respective antenna port of aplurality of antenna ports of the base station, wherein the plurality ofsignals include a plurality of beam reference signals, a plurality ofbeam refinement reference signals, a plurality of channel stateinformation reference signals (CSI-RSs), or a combination thereof; andperforming channel estimation for each beam of the plurality of beamsfrom the plurality of antenna ports based on the plurality of signals.2. The method of claim 1, wherein a millimeter wave (mmW) band is usedfor the wireless communications.
 3. The method of claim 1, wherein theplurality of beam reference signals are received during asynchronization subframe.
 4. The method of claim 1, wherein the channelestimation includes at least one of narrowband channel estimation orwideband channel estimation.
 5. The method of claim 1, furthercomprising: selecting one or more beams from the plurality of beamsbased on the channel estimation; and transmitting, to the base station,a feedback signal including information about the one or more beamsselected from the plurality of beams within one or more frequency bands.6. The method of claim 5, further comprising: ranking the plurality ofbeams based on the channel estimation, wherein the selecting the one ormore beams from the plurality of beams is based on the ranking.
 7. Themethod of claim 6, wherein the performing the channel estimation for theplurality of beams further comprises: performing wideband channelestimation for each beam of the plurality of beams, wherein the rankingis further based on wideband channel estimation for the plurality ofbeams.
 8. The method of claim 5, wherein the selecting the one or morebeams among the plurality of beams comprises: selecting at least onebeam with a high measurement of the channel estimation for each of aplurality of symbols, the high measurement being greater than athreshold measurement for the channel estimation, each symbol beingassociated with a set of beams from the plurality of antenna ports,wherein the one or more beams are selected among the beams with the highmeasurements for the plurality of symbols.
 9. The method of claim 8,further comprising: receiving a plurality of extended synchronizationsignals (ESSs), each ESS indicating a corresponding symbol of theplurality of symbols; and associating each set of beams of the pluralityof beams to a respective symbol of the plurality of symbols based on acorresponding ESS of the plurality of ESSs.
 10. The method of claim 8,wherein the one or more beams are selected within one or more frequencybands based on the channel estimation.
 11. The method of claim 5,wherein the feedback signal is transmitted to the base station via atleast one of a physical uplink control channel (PUCCH) or in uplinkcontrol information (UCI) conveyed via a physical uplink shared channel(PUSCH).
 12. The method of claim 5, wherein the feedback signal istransmitted to the base station through a random access channel (RACH)subframe.
 13. The method of claim 1, wherein the channel estimation isbased on a measurement of at least one of a signal-to-noise ratio, anantenna gain, or a reference signal measurement of each of the pluralityof beams.
 14. A method of wireless communication by a base station,comprising: transmitting, to a user equipment (UE), a plurality ofsignals through a plurality of beams of the base station, each of theplurality of beams corresponding to a respective antenna port of aplurality of antenna ports of the base station, wherein the plurality ofsignals include a plurality of beam reference signal, a plurality ofbeam refinement reference signals, a plurality of channel stateinformation reference signals (CSI-RSs), or a combination thereof; andreceiving, from the UE, a feedback signal including information aboutone or more beams selected from the plurality of beams within one ormore frequency bands.
 15. The method of claim 14, wherein a millimeterwave (mmW) band is used for the wireless communication.
 16. The methodof claim 14, where the plurality of beam reference signals aretransmitted during a synchronization subframe.
 17. The method of claim14, wherein the one or more beams are selected based on channelestimation for each beam of the plurality of beams and the channelestimation includes at least one of narrowband channel estimation orwideband channel estimation.
 18. The method of claim 14, wherein thetransmitting the plurality of signals includes: sweeping through aplurality of directions in different symbols to transmit the pluralityof signals.
 19. The method of claim 14, wherein the feedback signal isreceived from the UE via at least one of a physical uplink controlchannel (PUCCH) or in uplink control information (UCI) conveyed via aphysical uplink shared channel (PUSCH).
 20. The method of claim 14,wherein the feedback signal is received from the UE through a randomaccess channel (RACH) subframe.
 21. The method of claim 14, furthercomprising: performing communication with the UE based on the feedbacksignal.
 22. The method of claim 21, further comprising: choosing a beamamong the one or more beams based on the feedback signal, wherein thecommunication with the UE is performed via the chosen beam.
 23. A userequipment (UE) for wireless communication, comprising: a memory; and atleast one processor coupled to the memory and configured to: receive,from a base station, a plurality of signals through a plurality of beamsof the base station, each of the plurality of beams corresponding to arespective antenna port of a plurality of antenna ports of the basestation, wherein the plurality of signals include a plurality of beamreference signals, a plurality of beam refinement reference signals, aplurality of channel state information reference signals (CSI-RSs), or acombination thereof, and perform channel estimation for each beam of theplurality of beams from the plurality of antenna ports based on theplurality of signals.
 24. The UE of claim 23, wherein the at least oneprocessor is further configured to: select one or more beams from theplurality of beams based on the channel estimation; and transmit, to thebase station, a feedback signal including information about the one ormore beams selected from the plurality of beams within one or morefrequency bands.
 25. The UE of claim 24, wherein the at least oneprocessor is further configured to: rank the plurality of beams based onthe channel estimation, wherein the selecting the one or more beams fromthe plurality of beams is based on the ranking.
 26. The UE of claim 25,wherein the at least one processor configured to perform the channelestimation for the plurality of beams is configured to: perform widebandchannel estimation for each beam of the plurality of beams, wherein theranking is further based on wideband channel estimation for theplurality of beams.
 27. The UE of claim 26, wherein the at least oneprocessor configured to select the one or more beams among the pluralityof beams is configured to: select at least one beam with a highmeasurement of the channel estimation for each of a plurality ofsymbols, the high measurement being greater than a threshold measurementfor the channel estimation, each symbol being associated with a set ofbeams from the plurality of antenna ports, wherein the one or more beamsare selected among the beams with the high measurements for theplurality of symbols.
 28. A base station for wireless communication,comprising: a memory; and at least one processor coupled to the memoryand configured to: transmitting, to a user equipment (UE), a pluralityof signals through a plurality of beams of the base station, each of theplurality of beams corresponding to a respective antenna port of aplurality of antenna ports of the base station, wherein the plurality ofsignals include a plurality of beam reference signal, a plurality ofbeam refinement reference signals, a plurality of channel stateinformation reference signals (CSI-RSs), or a combination thereof, andreceiving, from the UE, a feedback signal including information aboutone or more beams selected from the plurality of beams within one ormore frequency bands.
 29. The base station of claim 28, wherein the atleast one processor is further configured to: performing communicationwith the UE based on the feedback signal.
 30. The base station of claim29, wherein the at least one processor is further configured to: choosea beam among the one or more beams based on the feedback signal, whereinthe communication with the UE is performed via the chosen beam.